Zimbra with Trend-Micro IMSS Mail gateay

Hello Guys,

I am setting up Zimbra in a mail box role and Mail gateway will probably be an Iron port appliance or Trend-Micro IMSS Spam filtering gateway. I would like to know since zimbra looks for domain's MX record for successful installation what changes I should make since MX record will be pointed to other appliance and not to this server.
Also, after installation how do I instruct zimbra for foreign domain mail delivery to this Gateway appliance?

Absolutely since IMSS is gonna be my mail gw inbound as well as outbound mail will be routed through it. Now the confusion here is, while installing we have to specify MX record to zimbra since that is a mandate. Now say this is my scenario MX record for exmaple.com is mail.example.com pointed to Public IP address which is IMSS say 20.20.20.20 natted to 192.168.1.10 and Zimbra sits on 192.168.1.15.
I would like to know in this situation what changes I should make on zimbra for successful email routing.